Stephen A. Sebo

Use of leakage currents of insulators to determine the stage characteristics of the flashover process and contamination level prediction

In order to improve the reliability of power transmission lines, one of the key issues is to reduce the hazard of contamination flashovers. Presently, the most efficient way is to clean (or replace) the heavily polluted insulators. The leakage current is the critical online quantity that can be detected. A number of laboratory experiments on 35 kV voltage class ceramic and glass insulators show that the leakage current fully reflects the entire process of contamination flashover development. The test results reveal that the process can be classified into three stages, i.e., security stage, forecast stage and danger stage. The results, that were duplicated several times, are based on three characteristics of the leakage current, i.e., the root-mean-square value, waveforms, and power spectrum estimation. In addition, the boundaries of the three stages in both time domain and power spectrum domain are also determined. All these can be used for the stage pre-warning of contamination flashovers. The security stage is most important since it precedes the contamination flashover sufficiently. The three characteristics of the leakage current in the security stage are proposed as the inputs of a neural network model together with the operating voltage, and the relative humidity in order to determine the equivalent salt deposit density (ESDD) of the insulators. The comparison of the simulated and actual (measured) results demonstrates that the ESDD prediction model has a very low relative error if the training data and the testing data both come from the security stage. The application of this research results in (1) optimal ESDD prediction inputs and (2) sufficient pre-warning time before the ultimate contamination flashover.

Zero-Sequence Current Distribution Along Transmission Lines

The zero-sequence current distribution among ground wires and the ground-return path of overhead transmission lines is subject to change in the vicinity of a ground fault and the feeding point. This paper introduces the concepts used in connection with the end-effect phenomena of transmission lines and describes the necessity for solving the end-effect current distribution. After dealing with the influential factors and basic assumptions, it describes a new calculation method which can be extended to develop generalized equations. The procedures to be followed in the case of ground faults supplied from either one or both sides are detailed. A summary of field tests is also included. The deviations between the measured and the computed values are found to be less than 15 percent. Several cases were studied to investigate the effect of factors, such as ground-wire material, tower-footing resistance, soil resistivity, and distance between feeding point and fault.

Power transformer resonance-measurements and prediction

The authors describe and summarize the results of impedance measurements of 12 converter transformers and 12 nonconverter transformers in the 50 Hz to 1 MHz frequency range. Impedance characteristics associated with various parameters are discussed. Studies of transformer winding first resonant frequencies and prediction equations are presented. Many conditions are represented: single and three-phase transformers, different core and winding designs, high voltage and low voltage windings, and various terminations. From measured driving point impedance, it can be clearly seen that the profile of impedance changes both in terms of magnitude and phase angle as a function of frequency. The first resonant frequency is found at the first zero crossing of the phase angle. The highest value found was 80 kHz. >

HDVC converter station tests in the 0.1 to 5 MHz frequency

Radiofrequency (RF) quantities in and around an HVDC converter station were measured in the 0.1 to 5 MHz frequency range to provide baseline data for the formulation of a means to predict the RF performance of such stations. Special records were taken at selected points and traverses in the station. The RF voltages of buses were measured and ground-level electric field strength and magnetic flux density measurements were conducted. To obtain the maximum amount of information about a converter pole when it is considered as an RF source, swept frequency (broadband) measurements were selected as the prime records for the analysis of pole performance. The principal source of the RF noise is the firing valve. Additional impedance versus frequency measurements of station components were also performed for undertaking RF modeling of the converter station. >

Modeling of converter transformers using frequency domain terminal impedance measurements

Frequency-dependent node-to-node impedance function (NIF) models of power system equipment based on systematic broad frequency range (50 Hz to 1 MHz) external driving point impedance measurements are described. Such models are needed to calculate and predict the radio frequency electromagnetic (EM) noise produced by the valve ignition of a converter station. The application of the transformer frequency-domain NIF model related to HVDC converter station EM noise calculations is demonstrated. Calculated performance data are compared with field measurements. >

Measurement of the frequency dependent impedance of major station equipment

High-voltage direct-current (HVDC) converter stations generate radio frequency (RF) electromagnetic (EM) noise due to valve firing. The noise propagates into the AC and DC switchyards and along their corresponding transmission lines. This noise can affect the performance of adjacent communication, control, and computer equipment, and it can interfere with carrier system operation. Therefore, it is important to measure, predict, and mitigate the EM noise and interference. Measurements on equipment are necessary for the purpose of determining these impedance characteristics. A description is given of the instrumentation, improved measurement procedures, a systematic measurement program, and equipment representation concepts. All these have been developed and applied successfully in practice in the course of a project sponsored by the Electric Power Research Institute (EPRI). Transformer-related examples are used to illustrate the more relevant features of the study. >

Electric field and potential distributions along dry and clean non-ceramic insulators

The electric field and potential distributions in the vicinity of non-ceramic insulators under dry and clean conditions are presented. A three-dimensional electric field analysis program, COULOMB, has been used for the calculations. A three-phase 765 kV power line tower geometry is considered for the potential distribution calculations along the insulators. For three-phase energization, two-dimensional contours of the three-dimensional equipotential surfaces are presented in selected vertical planes. The effects of the presence of power line conductors and of the three phase vs. single phase energization on the electric field and potential distributions have been investigated.

Partial discharge measurements in air and argon at low pressures with and without a dielectric barrier

Partial discharge (PD) characteristics in air and argon under low pressures down to 13.3 Pa (0,1 Torr) and 60 Hz AC energization are studied in an energized needle-plane electrode arrangement. The electrode configuration, vacuum chamber, facilities, and electrical connections for the experimental setup are described. Two cases are studied for each of two gases, air and argon, with 20 mm spacing between the two electrodes: (1) with and (2) without a Teflon/spl reg/ cap (dielectric barrier). Results for the four series of experiments and analysis of the discharge current pulse waveforms are presented. Topics discussed are the typical waveforms of the discharge current pulses at different pressures, and discharge current pulse rise time vs. pressure relationships.

Calculation of single phase AC and monopolar DC hybrid corona effects

Operating a hybrid HVAC and HVDC power transmission line is an option for increasing the efficiency of power transmission and overcoming the difficulties in obtaining a new right-of-way. This paper proposes a new calculation method for the study of hybrid power line corona. The proposed method can be used to calculate DC corona losses and corona currents in DC or AC conductors for single phase AC and monopolar DC hybrid lines. Profiles of electric field strength and ion current density at ground level can be estimated. The effects of the presence of an energized AC conductor on DC conductor corona and DC voltage on AC conductor corona are included in the method. Full-scale and reduced-scale experiments were utilized to investigate the hybrid line corona effects. Verification of the proposed calculation method is given.

Maximum likelihood estimation of transformer high frequency parameters from test data

A methodology for the development of transformer high-frequency models is presented. Time constants of the transformer transfer function are estimated from frequency response measurements by using the nonlinear least-squares method in the frequency domain. Then, the time response of transfer functions is used in conjunction with the maximum-likelihood method to estimate the transformer model parameters. As a case study, a high-frequency model of a 15 kVA, 7620/240 V single-phase distribution methodology. >